The present invention relates to edge guides which perform a heat sink function with respect to components mounted on a circuit board. More particularly, the present invention relates to edge guides which are attached to a heat sink plate on which a printed circuit board is mounted and which are used to provide a heat flow path from components of the circuit board to a chassis within which the board is mounted and to aid and provide stability in the mounting of the circuit board within the chassis.
In the mounting of circuit boards, it is usually essential to provide a heat sink for the electronic components borne by the boards. A typical heat sink is a piece of metal or some other heat conducting material that effectively draws heat away from the circuit board components and transfers this heat to the surrounding air. Heat sinks are often required because many electronic components (especially high power electronic components) tend to malfunction if the temperatures associated therewith rise above certain rated values. Since circuit boards are often mounted within small chassis enclosures in order to save space, it is common practice to provide a heat flow path between the chassis and the circuit components so that heat from such components can be radiated into the air surrounding the chassis.
However, as chassis enclosures become smaller and smaller in accordance with recent design trends, the problem of heat removal becomes more complex. More specifically, when heat-emanating components are densely packed within a small enclosure, heat within the enclosure builds up and therefore more efficient heat sink devices become necessary. Additionally, the use of small enclosure chassis results in circuit board mounting difficulties as the dense packing of the circuit boards within such chassis often leaves little room for adequate fastening of each of the circuit boards at a plurality of fastening points.
Heat removing edge guides have previously been used in an attempt to overcome the above-mentioned problems. Such guides are attached to heat sink plates upon which circuit boards ar mounted and provide an additional heat flow path between the circuit board components and the chassis or a cold wall support within the chassis in order to facilitate the removal of heat from the circuit board components. Additionally, the previously used heat removing edge guides aid in the mounting of the boards since the fastening of such boards requires only the simple rotation of easily accessible screws corresponding to the edge guides. A rotation of a screw associated with an edge guide causes the upper surface of a moving portion of that edge guide to contact a fin extending from the wall support and located adjacent to the edge guide in order to fasten or lock a particular board into place and to provide an additional heat flow path for each of the circuit components.
For example, U.S. Pat. No. 4,318,157 shows a device for removably mounting a printed circuit board to a chassis. An elongated shaft in the form of a jackscrew carries a plurality of ramps so as to be movable laterally thereof. A forcing mechanism on the shaft urges the ramps laterally in opposite direction and forces at least one of the ramps and the circuit board assembly against opposite wall portions of one of the channels in the chassis. Each of the intermediate ramps located between a first ramp and a bracket is formed of a heat conductive material and is trapezoidal in shape. These intermediate ramps are solid except for a central, longitudinal bore to accommodate the jackscrew. This is a relatively complex arrangement which requires multiple movable ramps and brackets to obtain the lateral movement. In addition, the movable ramps do not provide a continuous contact along the wall portions which decreases the heat transfer efficiency of the apparatus. Because there are multiple mating surfaces, uniform contact pressure cannot be assured.
Another retaining apparatus is shown in U.S. Pat. No. 4,819,713. Here, however, the ramps or wedges are not solid in order to be returned on the rail, in addition to constituting multiple ramps which is a more complex structure and is not as heat removal efficient as desired. The U-shaped structure of the wedges represents a very narrow surface which means a high thermal resistance and lower efficiency.
Although the previously used edge guides perform useful functions, they are still considered to provide less than satisfactory results. For example, heat removal through the additional heat flow path is nearly nonexistent due to the relatively small surface area of the adjacent fin surface contacting the upper surface of the moving portion. Since the surface area of the upper surface of the moving portion is relatively small due to its short length and width, heat removal through the upper surface of the moving portion and the adjacent fin is limited. Also, the short length and width of each moving portion results in less contact area between the upper surfaces of the moving portions and the fins adjacent thereto so that the pressure caused by rotation of each of the screws is focused on a smaller area and mounting stability is sacrificed in the edge guide system.